Designing an exhaust is a job. While it is way simpler with a turbo engine (no harmonics to account for), it still is not that simple.
For what its worth, here are some infos i came across when designing mine.
-dont fall for "turbo" or "vortex" mufflers, that is just marketing.
- straight through mufflers are somewhat good for med / high frequency sounds only, and bad for low frequencies. They do not limit flow that much, and act like a cone.
- reflective mufflers are very good at muffling low/med frequencies, but they limit flow. To counter that, just get a bigger one...
- problem: there is not a lot of room to install a small additionnal muffler, so you wont be able to fit a big one.
-isis does not make exhaust, they rebrand chinese crap. Meaning your current boxes are called mufflers, but dont really act like ones.
- the bigger in diameter, the more dB they will absorb.
- the attenuation gets on a broader band when the muffler is longer; at the same time, the absorbed sound is decreased.
- the attenuation band depends on diameter to length ratio;
- you can also use quarter wavelength tuning to remove just a small portion of sound; it wont hinder the flow, but it will only work for a very small frequency band. This is typically used to remove incabin drone. Comes as standard on S2000 exhaust for example.
Calculator is here:
Quarter Wave Tube - DiracDelta Science & Engineering Encyclopedia
Basically, if you really want it silent... get a quality, silent exhaust. A good muffler cost 200+ $, you need at least 2 of them + labor. You can get magnaflow, or vibrant performance, they do sell some good stuff.
I was thinking about HKS silent power, for example. These are really straight fit exhausts, i could not believe it when i saw my tuner install one.